We selected a group of children with unilateral hydroureteronephrosis to assess the significance of the differential renal function as determined by nuclear renography as a factor in defining obstructive dilatation. All children who presented to our institution during a 21-month period with unilateral hydroureteronephrosis and a normal contralateral kidney were reviewed. Patients with reflux or bladder pathology were excluded, resulting in 13 evaluable patients younger than age 2 years. Diuretic renography was performed using 99mtechnetium-diethylenetriaminepentaacetic acid in accordance with the well tempered renogram. Ancillary studies included voiding cystourethrography, ultrasonography and/or excretory urography. Of the 13 patients 5 had a differential function greater than 55% in the hydronephrotic kidney. Ten of the 13 patients, including the 5 with supranormal function, underwent surgical correction in the form of ureteral reimplantation for recurrent infections, increasing hydronephrosis, decreasing or decreased function and loss of parenchyma. Stenotic distal adynamic ureteral segments with proximal dilatation were confirmed in all cases. The implication by nuclear renography that a dilated kidney has better function than its morphologically normal counterpart is at variance with any available clinical or experimental data. The etiology of this apparently elevated function is unclear and its presence on a renographic study should not be interpreted as suggestive that the kidney is free of risk of nephron loss. Management decisions should not be based solely on the differential function as determined by nuclear renography.